The present invention relates to methods and apparatus for imaging three-dimensional objects. The invention is particularly useful in angiography for imaging a person""s vascular system, and is therefore described below with respect to this application.
Electromagnetic radiation has long been used to produce images of internal structures of the human body for purposes of diagnosis or treatment of diseases. One technique which has gained widespread use since the 1970""s is computerized tomography (CT), sometimes called computerized axialtomography (CAT), in which a narrow beam of X-rays is swept across an area of the body and is recorded by a radiation detector as a pattern of electrical impulses, while a computer is used to integrate the data for many such sweeps and to reconstruct a three-dimensional image of the examined volume. In the current CT systems, the radiation source is generally a fan-shaped beam, and the radiation detector includes a line of detector elements aligned with the fan-shaped beam so as to detect the level of the radiation after having traversed the object being examined. The body is exposed to the radiation at a plurality of different image planes (slices), and at a plurality of different angular positions for each image plane, while a large number of gray levels of radiation are sensed by each detector element. For example, a typical procedure may involve in the order of 256 slices, each at 128 different angular positions, with each detector element recording up to 128 gray levels. Many reconstruction algorithms are known for reconstructing the three-dimensional image from this data, but nevertheless, because of the vast amount of data needed, the reconstruction procedure is done off-line, rather than in real-time on-line.
Angiography, on the other hand, involves the radiographic examination of arteries and veins, in which a contrast medium is injected into the vascular system to cause a denser shadow than would be caused by other tissues, thereby enabling the blood vessels to be distinguished from the other tissues. In digital subtraction angiography (DSA), an X-ray image (called a; xe2x80x9cmasking imagexe2x80x9d) of the patient is taken before the contrast material is injected; another X-ray image (called a xe2x80x9ccontrast imagexe2x80x9d) is taken after the contrast material is injected; and the masking image is subtracted from the contrast image to leave only the DSA image of the blood vessels enabling them to be distinguished from the other tissue. Examples of known DSA systems are described in U.S. Pat. Nos. 5,630,414, 6,052,476, and 6,118,845, the disclosures of which are incorporated herein by reference.
At the present time, angiography is more widely used for producing a two-dimensional image, rather than a three-dimensional image, of the arteries and veins under examination. Making a three-dimensional examination using existing techniques would involve very costly equipment and/or substantial processing time so as to effectively preclude obtaining the results substantially at the time of the examination. As a result, it is necessary to search for the best angle for the two dimensional examination. This can be imprecise and somewhat awkward particularly since the examination involves the injection of the contrast material.
Providing angiographic images in three-dimensions, and in real-time with the performance of the diagnostic or treatment procedures, would substantially aid in reducing the dose levels and contrast media loads needed during the diagnostic or treatment procedure. Thus, providing the physician with three-dimensional angiographic views of the patient""s vascular system in substantially real-time would enable the physician to analyze the three-dimensional angiography image directly, to decide on the best treatment strategy, and to determine the best projection angle for the treatment, such as the positioning of catheters, coils, balloons or stents.
In addition, the value of the images produced by DSA for diagnostic or treatment purposes depends to a high degree on the contrast-to-noise ratio (CNR) of the DSA image. The CNR of an image is to be distinguished from the signal-to-noise ratio (SNR), and is generally defined as being the difference in the SNRs of adjacent imaged regions. Thus, enhancing the CNR of a DSA image would better enable the physician to discern details of the patient""s vascular system and therefore better enable the physician to utilize this information for diagnostic or treatment purposes.
An object of the present invention is to provide a method and apparatus for producing a three-dimensional image of a body in real time. Another object of the invention is to provide a method and apparatus for producing three-dimensional angiographic images in real time to better enable diagnosis or treatment of a disease in the human body. A further object of the invention is to provide a method and apparatus for enhancing the CNR of a DSA image.
According to one aspect of the present invention, there is provided a method of imaging a body, comprising the steps: (a) exposing the body from one angular position with respect to the body, to radiation transmitted through the body from a radiation source on one side of the body to a radiation detector in alignment with the radiation source on the opposite side of the body; the radiation detector including a two-dimensional matrix of detector elements producing electrical outputs corresponding to the magnitudes of the radiation received by each detector element; the radiation source producing a conical beam sufficiently large to cover all the detector elements in the two-dimensional array after traversing the body; (b) successively changing the angular position in a plurality of angular increments over a predetermined angular sector, and repeating the exposing step at each of the angular positions; (c) storing the electrical outputs of each of the detector elements in each of the angular positions; and (d) utilizing the stored outputs for reconstructing and displaying the image of the body in three dimensions.
According to another aspect of the present invention, there is provided apparatus for imaging a body, comprising: a support for the body to be imaged; a radiation source at one side of the body to be imaged, the radiation source producing a conical beam to be transmitted through the body; a radiation detector at the opposite side of the body to be imaged and aligned with the radiation source; the radiation detector including a two-dimensional matrix of detector elements each producing an electrical output corresponding to the magnitude of the radiation received by the detector element; the conical beam produced by the radiation source being sufficiently large to cover all the detector elements in the two-dimensional array; a drive for effecting relative rotation, to a plurality of different angular positions, between the body on the one hand and the radiation source and radiation detector on the other hand, while keeping the radiation detector aligned with the radiation source; a computer for receiving the electrical outputs of each of the detector elements in each of the plurality of different angular positions, and for reconstructing the image of the body in three dimension; and a display for displaying the image of the body in three dimensions or in a selected plane.
According to further features in one described preferred embodiment, the electrical output of each detector element is digitized to binary values according to whether the magnitude of the radiation received by the respective detector element is above or below a predetermined threshold. Before the successive exposures, a contrast material is introduced into the body, which results in a first binary value (e.g., xe2x80x9c0xe2x80x9d) being produced when the contrast material is not in the path of the radiation to the detector element, and a second binary value (e.g., xe2x80x9c1xe2x80x9d) being produced when the contrast material is in the path of the radiation to the detector element.
According to one described preferred implementation, the stored outputs of the detector elements are utilized for reconstructing and displaying the image of the body in three dimensions by dividing the exposed volume into a three-dimensional array of voxels, and comparing the stored outputs of the detector elements for each of the voxels with look-up tables previously stored in the computer.
According to a second described preferred implementation, the stored outputs are utilized for reconstructing and storing the image of the body in three dimensions by dividing the exposed volume into a three-dimensional array of voxels, and utilizing the voxels in which the second value (e.g., xe2x80x9c1xe2x80x9d) is present for all angular positions of exposure of the body for reconstructing and storing the image of the body in three dimensions. In this described implementation, the angular position of the radiation source and the radiation detector is changed in angular increments (of xcex8) around the longitudinal axis (e.g., the Z-axis) of the body being imaged. Also, for better resolution, the angular position of the radiation source and the radiation detector is also changed in angular increments (of xcex2) around another axis (e.g., the X-axis) perpendicular to the longitudinal axis of the body being imaged.
Because in both of the above implementations of this described preferred embodiment, the apparatus includes a two-dimensional array of detector elements (rather than a line as in conventional CT apparatus), and because the apparatus needs to make only a few series of exposures at different angular positions (rather than a large number of such series of exposures or slices at different axial positions as in CT examination), and further, because the computer needs to distinguish between only two levels of data from each detector element (rather than many gray levels as in CT examination), this embodiment of the present invention can produce a three-dimensional image of the volume under examination in virtually real-time, as compared to existing techniques. For example, if exposures at 128 angular increments are made along only a few axes of the examined body, at a rate of 25 exposures per second, the exposure process would take only about 5 seconds per axis. Such a method and apparatus are therefore particularly useful in producing angiographs of a patient""s vascular system in a virtually real-time manner and on-line with the diagnosis or treatment of the patient.
According to a still further aspect of the present invention, there is provided a method, and apparatus, for angiographically imagining a portion of a patient""s vascular system to enhance the CNR of the produced image. Briefly, this is done by: producing a sequence of masking images of the portion of the patient""s vascular system; determining the masking image in the sequence having a minimum change over its immediately preceding masking image in the sequence; injecting a contrast material into the patient""s vascular system; producing a corresponding sequence of contrast images of the portion of the patient""s vascular system while containing the contrast material; and subtracting from each contrast image of the sequence the corresponding masking image in the sequence starting with the minimum change masking image, to thereby produce a difference image having an enhanced contrast-to-noise ratio (CNR).
Further features and advantages of the invention will be apparent from the description below.
Such a method and apparatus can be used for both two-dimensional angiography as well as for three-dimensional angiography.